Implantable neurostimulation systems have proven therapeutic in a wide variety of diseases and disorders. Pacemakers and Implantable Cardiac Defibrillators (ICDs) have proven highly effective in the treatment of a number of cardiac conditions (e.g., arrhythmias). Spinal Cord Stimulation (SCS) systems have long been accepted as a therapeutic modality for the treatment of chronic pain syndromes, and the application of tissue stimulation has begun to expand to additional applications such as angina pectoralis and incontinence. Deep Brain Stimulation (DBS) has also been applied therapeutically for well over a decade for the treatment of refractory chronic pain syndromes, and DBS has also recently been applied in additional areas such as movement disorders and epilepsy. Further, in recent investigations, Peripheral Nerve Stimulation (PNS) systems have demonstrated efficacy in the treatment of chronic pain syndromes and incontinence, and a number of additional applications are currently under investigation. Furthermore, Functional Electrical Stimulation (FES) systems, such as the Freehand system by NeuroControl (Cleveland, Ohio), have been applied to restore some functionality to paralyzed extremities in spinal cord injury patients.
These implantable neurostimulation systems typically include one or more electrode carrying stimulation leads, which are implanted at the desired stimulation site, and a neurostimulator (e.g., an implantable pulse generator (IPG)) implanted remotely from the stimulation site, but coupled either directly to the stimulation lead(s) or indirectly to the stimulation lead(s) via a lead extension. The neurostimulation system may further comprise an external control device, such as a handheld patient programmer, to remotely instruct the neurostimulator to generate electrical stimulation pulses in accordance with selected stimulation parameters.
Electrical stimulation energy may be delivered from the neurostimulator to the electrodes in the form of an electrical pulsed waveform. Thus, stimulation energy may be controllably delivered to the electrodes to stimulate neural tissue. The combination of electrodes used to deliver electrical pulses to the targeted tissue constitutes an electrode combination, with the electrodes capable of being selectively programmed to act as anodes (positive), cathodes (negative), or left off (zero). In other words, an electrode combination represents the polarity being positive, negative, or zero. Other parameters that may be controlled or varied include the amplitude, width, and rate of the electrical pulses provided through the electrode array. Each electrode combination, along with the electrical pulse parameters, can be referred to as a “stimulation parameter set.”
With some neurostimulation systems, and in particular, those with independently controlled current or voltage sources, the distribution of the current to the electrodes (including the case of the neurostimulator, which may act as an electrode) may be varied such that the current is supplied via numerous different electrode configurations. In different configurations, the electrodes may provide current or voltage in different relative percentages of positive and negative current or voltage to create different fractionalized electrode configurations.
As briefly discussed above, a hand-held programmer can be used to instruct the neurostimulator to generate electrical stimulation pulses in accordance with the selected stimulation parameters. Typically, the stimulation parameters programmed into the neurostimulator can be adjusted by manipulating controls on the hand-held programmer to modify the electrical stimulation provided by the neurostimulator system to the patient. However, the number of electrodes available, combined with the ability to generate a variety of complex stimulation pulses, presents a huge selection of stimulation parameter sets to the clinician or patient.
To facilitate such selection, the clinician generally programs the neurostimulator through a computerized programming system. This programming system can be a self-contained hardware/software system, or can be defined predominantly by software running on a standard personal computer (PC). The PC or custom hardware may actively control the characteristics of the electrical stimulation generated by the neurostimulator to allow the optimum stimulation parameters to be determined based on patient feedback or other means and to subsequently program the neurostimulator with the optimum stimulation parameter set or sets, which will typically be those that stimulate all of the target tissue in order to provide the therapeutic benefit, yet minimizes the volume of non-target tissue that is stimulated. The computerized programming system may be operated by a clinician attending the patient in several scenarios.
For example, in order to achieve an effective result from SCS, the lead or leads must be placed in a location, such that the electrical stimulation will cause paresthesia. The paresthesia induced by the stimulation and perceived by the patient should be located in approximately the same place in the patient's body as the pain that is the target of treatment. If a lead is not correctly positioned, it is possible that the patient will receive little or no benefit from an implanted SCS system. Thus, correct lead placement can mean the difference between effective and ineffective pain therapy. When electrical leads are implanted within the patient, the computerized programming system, in the context of an operating room (OR) mapping procedure, may be used to instruct the neurostimulator to apply electrical stimulation to test placement of the leads and/or electrodes, thereby assuring that the leads and/or electrodes are implanted in effective locations within the patient.
Once the leads are correctly positioned, a fitting procedure, which may be referred to as a navigation session, may be performed using the computerized programming system to program the external control device, and if applicable the neurostimulator, with a set of stimulation parameters that best addresses the painful site. Thus, the navigation session may be used to pinpoint the stimulation region or areas correlating to the pain. Such programming ability is particularly advantageous for targeting the tissue during implantation, or after implantation should the leads gradually or unexpectedly move that would otherwise relocate the stimulation energy away from the target site. By reprogramming the neurostimulator (typically by independently varying the stimulation energy on the electrodes), the stimulation region can often be moved back to the effective pain site without having to re-operate on the patient in order to reposition the lead and its electrode array. When adjusting the stimulation region relative to the tissue, it is desirable to make small changes in the proportions of current, so that changes in the spatial recruitment of nerve fibers will be perceived by the patient as being smooth and continuous and to have incremental targeting capability.
Electrical stimulation energy may be delivered from the neurostimulator to the electrodes using one or more current-controlled sources for providing stimulation pulses of a specified and known current (i.e., current regulated output pulses), or one or more voltage-controlled sources for providing stimulation pulses of a specified and known voltage (i.e., voltage regulated output pulses).
For example, the Precision® neurostimulator, marketed by Boston Scientific Neuromodulation Corporation, has a constant current source hardware platform with sixteen independent current sources that can independently deliver constant current at different magnitudes to any combination of electrodes over multiple channels. As another example, the Bion® microstimulator, marketed by Boston Scientific Neuromodulation Corporation, has a simpler, but smaller, constant current source hardware platform that can deliver current at equal magnitudes between two electrodes over a single channel. The Synergy® and Restore® neurostimulators, marketed by Medtronic, Inc., deliver electrical energy at a constant voltage, with both neurostimulators having a single voltage source at any point time. The Genesis® and EON® neurostimulators, marketed by Advanced Neuromodulation Systems, have single constant current sources.
In single source systems, whether current or voltage regulated, the spatial recruitment of nerve fibers using stimulation pulses is subject to variations of current-to-voltage relationships (i.e., impedance of the tissue, electrode, and electrode-tissue interface), since the electrical current cannot be adjusted amongst multiple electrodes in response to such current-to-voltage relationship variations. With respect to multiple source systems, the spatial recruitment of nerve fibers using voltage regulated output pulses is more subject to current-to-voltage relationship variations than the spatial stimulation of nerve fibers using current regulated output pulses. In particular, when output pulses are current regulated on each active electrode, the current is automatically maintained on the respective active electrode regardless of impedance variations. Thus, because current, as opposed to voltage, is most directly related to stimulation strength, the use of current regulated output pulses reduces the sensitivity of the spatial recruitment of nerve fibers to impedance variations.
This is not the case, however, when output pulses are voltage regulated, since the current on each active electrode will vary with the change in impedance. Even a small change in the current distribution on the active electrodes can change the spatial recruitment of nerve fibers causing a reduction in therapeutic efficacy and/or patient comfort. In part, this is because the clinical usage range for stimulation (difference between perception and maximum tolerated amplitude) is only a fraction of the therapeutic stimulation amplitude.
With respective to system having multiple sources, changes in impedance will cause changes in the distribution of current among electrodes in a voltage regulated system even if each electrode has a dedicated voltage regulated output. These impedance changes can occur over time as the electrodes encapsulate or in the short term as a result of the encapsulation process, changes at the electrode-tissue interface, patient movement, respiration, arterial perfusion, postural changes. Thus, without some means of adjusting the output voltage distribution on all active electrodes based on impedance variations, the stimulation pattern can change, resulting in reduced therapeutic efficacy.
A method and means for adjusting the voltages on all active electrodes to generate a desired current distribution on the active electrodes would help maintain therapy in the presence of impedance changes and allow new distributions of current to be generated when adjusting the stimulation region relative to the tissue.